RF power amplifiers are used in a variety of applications such as base stations for wireless communication systems etc. The signals amplified by the RF power amplifiers often include signals that have a high frequency modulated carrier having frequencies in the 400 megahertz (MHz) to 4 gigahertz (GHz) range. The baseband signal that modulates the carrier is typically at a relatively lower frequency and, depending on the application, can be up to 300 MHz or higher.
A device package for an RF power amplifier can include a transistor die (e.g., MOSFET (metal-oxide semiconductor field-effect transistor), LDMOS (laterally-diffused metal-oxide semiconductor), HEMT (high electron mobility transistor) along with an input and output impedance matching circuit incorporated therein. The input and output impedance matching circuits typically include LC networks that provide at least a portion of an impedance matching circuit that is configured to match the impedance of the transistor die to a fixed value. The device package may also include tuning circuits that are configured to filter out higher order harmonic components of the fundamental frequency to improve amplifier efficiency.
RF power architectures within the telecommunications field focus on achieving high DC-to-RF efficiency at significant power back-off from Psat (the average output power when the amplifier is driven deep into saturation). This is due to the high peak to average ratio (PAR) of the transmitted digital signals such as W-CDMA (wideband code division multiple access), LTE (long term evolution) and WiMAX (worldwide interoperability for microwave access).
One popular power amplifier architecture currently employed is the Doherty amplifier. The Doherty amplifier was first proposed by William H. Doherty, in 1936, and is described in “A new high efficiency power amplifier for modulated waves,” Proc. IRE, vol. 24, pp. 1163-1182, September 1936, the content of which is incorporated by reference in its entirety. The Doherty amplifier employs a main amplifier which provides amplification at all power levels, and a peaking amplifier, which turns on at a high power level. Efficiency is enhanced through load modulation of the main amplifier from the peaking amplifier.
Another popular power amplifier architecture currently employed is the Chireix amplifier. The Chireix amplifier was first proposed by H. Chireix in 1935, and is described in “High power outphasing modulation,” Proc. IRE, vol. 23, no. 11, pp. 1370-1392, November 1935, the content of which is incorporated by reference in its entirety. A Chireix amplifier utilizes an outphasing technique to amplify two phase-shifted constant envelope signals. The Chireix amplifier offers highly efficient and linear amplification without distortion.
Although the Doherty amplifier and the Chireix amplifier each offer high back-off efficiency for amplitude modulated signals, each design has limitations. In general, the Chireix amplifier provides higher efficiency than a Doherty amplifier in back-off ranges that are close to peak power, but less efficiency than a Doherty amplifier at lower power levels. Moreover, the Doherty amplifier typically has an efficiency droop (i.e., a region of lesser efficiency between two peaks) in the high power back-off region, whereas a Chireix amplifier offers more linear efficiency in the high power back-off region. Although some of these drawbacks of each amplifier can be addressed using additional circuitry and/or control techniques, known solutions substantially increase cost and circuit complexity and have limited success.